1. Field of the Invention
The invention relates to a cladding alloy powder for cladding the surface of a base material and to an alloy-clad member and an engine valve that use this cladding alloy powder. The invention more particularly relates to a cladding alloy powder that is well suited for high-temperature environments and to an alloy-clad member and an engine valve that use this cladding alloy powder.
2. Description of the Related Art
Heat-resistant steel is used for machinery that is employed in high-temperature atmospheres, e.g., the intake valves and exhaust valves of internal combustion engines, in order to improve, for example, the wear resistance. In particular, the valve face of the engine valve and the like must exhibit wear resistance, heat resistance, thermal impact resistance, and a low capacity to damage mating members, over a wide temperature range from ambient temperature to high temperatures. Accordingly, the heat-resistant steels typically used as a valve material do not adequately exhibit these properties. As a consequence, these properties are provided by melting a cladding powder alloy that has these properties and cladding (overlaying) this on the valve face. In the particular case of engines fueled by Compressed Natural Gas (CNG), due to the weak oxidizing power of the combustion atmosphere, a cladding (overlaying) alloy powder is used that exhibits excellent oxidation properties and that readily produces an oxide film.
Such a cladding alloy powder has been introduced, for example, as a cladding alloy powder including 20 to 60 mass % Mo, 0.2 to 3 mass % C, 5 to 40 mass % Ni, and 0.1 to 10 mass % Cr with the balance being Fe and unavoidable impurities (refer, for example, to Japanese Patent No. 2970670).
Due to the limitation on the Cr content, the cladding alloy region (referred to below as the cladding alloy) of an alloy-clad material that has been clad with such a cladding alloy powder exhibits a satisfactory production of molybdenum oxide, which exhibits the most effective solid lubricity. This molybdenum oxide functions as a solid lubricant, which results in the generation of excellent properties, i.e., a high wear resistance and a relatively low capacity to damage mating members.
However, the cladding alloy described in Japanese Patent No. 2970670, while being able to provide the properties described above, does not have a satisfactory toughness, and cracking due to, for example, thermal impact, may be caused. The reason for this is believed to be as follows: the hardness of the cladding alloy is increased due to the presence in the cladding alloy of coarse primary carbides of, for example, Mo, but these primary carbides form a fracture origin and thereby lower the toughness of the cladding alloy.
In addition, these coarse primary carbides have a shape that results in a high capacity to damage mating members and end up gouging the mating member material that comes into contact with the clad member, which also results in wear of the mating member material.